1. Field of the Invention
The present invention generally relates to a method and apparatus for image forming and more particularly, to a method and apparatus for image forming, which is capable of effectively collating a stack of single-sided or double-sided recording sheets in increasing order of page numbers in a desired ejection tray.
2. Discussion of Background
Some conventional image forming apparatuses, such as copying machines, printers, facsimiles, and other similar devices, are capable of performing a double-sided recording operation for recording on both the first and second sides of a recording sheet. Typically, in the double-sided recording operation of these conventional machines, a first toner image is formed on the first side of a recording sheet and a second toner image is formed on the second side of the same recording sheet after the first toner image is fixed. Before the second toner image is formed, the recording sheet is reversed. After completion of the second image forming operation, the second toner image is fixed and, as a result, a double-sided recording is achieved.
Various problems may occur with the above-described double-sided recording operation, such as difficulty in reversing the recording sheet, unstable sheet transfer due to a paper curl problem caused by the first fixing process, and other similar problems.
On the other hand, each of Japanese Unexamined Patent Application Publication Nos. 1-209470 (hereinafter “JP'470”) and 10-142869 (hereinafter “JP 869”) describes an image forming apparatus which transfers toner images to both the first and second sides of a recording sheet with two image carrying members and fixes the toner images via an one time fixing process.
In the image forming apparatus of JP '470, a first toner image is formed on a photoconductive member and is transferred onto a transfer belt with a first transfer member. Then, a second toner image is formed on the photoconductive member and is transferred onto a first surface of a recording sheet with the first transfer member. After that, the first toner image on the transfer belt is transferred onto the second side of the recording sheet with a second transfer member. Thereby, the first and second toner images are transferred onto both the first and second sides of the recording sheet and the recording sheet is then subjected to a fixing process.
Japanese Unexamined Patent Application Publication No. 3-253881 (hereinafter “JP'881”) describes an image forming apparatus which is similar to the image forming apparatus of JP '470. A difference between the two image forming apparatuses is that the image forming apparatus of JP '881 eliminates the second transfer member by reversing a polarity of the second toner image on the photoconductive member before it is transferred to the recording sheet. Thereby, the first and second toner images are transferred onto both the first and second sides, respectively, of the recording sheet without the second transfer member.
The image forming apparatus of JP '869 is provided with two transfer members and performs an one time fixing operation for fixing color images which have been transferred onto both the first and second sides of a recording sheet with the first and second transfer members, respectively. This image forming apparatus uses a guide member, which includes a flat plate on which a plurality of star-like wheels are rotatably mounted, to smoothly transfer the double-sided recording sheet with the color images on both the first and second sides thereof.
However, the image forming apparatuses, of JP '470, JP '869, and JP '881, have a drawback with respect to the stack order of the output recording sheets. For example, as shown in FIG. 1, a stack of recording sheets are output face down in a tray T1, but are face up in a tray T2. Therefore, the stack of recording sheets output to the trays T1 and T2 are different in page number order.
Japanese Unexamined Patent Application Publication No. 2000-19799 (hereinafter “JP'799”) describes an image forming apparatus that includes a tray switching mechanism for switching between face-down and face-up ejection trays. A sheet reverse mechanism is used to make it possible to switch the ejection trays without changing relationships between images and surfaces of a recording sheet. A technique is also used in which the relationships between images and surfaces of a recording sheet are changed when the ejection trays are switched from one to the other in an image forming apparatus having no sheet reverse mechanism.
However, JP '799 has a drawback in that a thick sheet is not properly transferred. This is because a recording sheet is bent when it is reversed by the sheet reverse mechanism.
In a case where an image forming apparatus, having a plurality of ejection trays as shown in FIG. 1, applies the technique for changing the relationships between images and surfaces of a recording sheet as described in JP '799, it is not easy for a user to properly instruct the change of the relationships between images and surfaces of a recording sheet.
Japanese Unexamined Patent Application Publication No. 2000-38234 (hereinafter “JP'234”) describes an image forming apparatus which reads images on both the first and second sides of a double-sided original and records the read images on both the first and second sides, respectively, of a recording sheet. In this apparatus, the double-sided original is reversed after a completion of reading the first side of the original and then, the second side of the original is read. In the recording operation, the recording sheet is reversed after the recording of a first image on a first side of the recording sheet is completed and then, a second image is recorded on the second side of the recording sheet.
Japanese Unexamined Patent Application Publication No. 11-258864 describes an image forming apparatus which reads images on both the first and second sides of a double-sided original by moving the original and then recording the read images on both the first and second sides of a recording sheet via a one time sheet transferring process.
However, the apparatuses which are capable of double-sided reading and double-sided recording have the drawback that they do not collate a stack of recording sheets, particularly when a plurality of ejection trays are used, and this drawback is in addition to the problems they have in common with conventional apparatuses (i.e., low sheet transfer stability and larger machine size).